In general, technology for wireless communication in close proximity is referred to as near-field communication, and its representative example is device-to-device (D2D) communication that enables direct communication between devices. In D2D communication, user equipment (UE) first discovers a counterpart UE with which the UE attempts to communicate, establishes a communication session through a base station (e.g., evolved NodeB (eNB)), and directly transmits traffic through the communication session to the counterpart UE to which the UE desires to transmit traffic. As such, guarantying efficient data transmission between neighbor UEs while addressing the overload of eNB, D2D communication is recently gaining popularity as a next-generation mobile communication technology.
A Long Term Evolution (LTE) system-based UE may operate in both cellular mode and D2D mode considering the communication environment of the site where the UE is located or the type of service that the UE desires to receive. Here, cellular mode denotes a mode in which the UE performs communication through an eNB, and D2D mode denotes a mode in which the UE directly performs D2D communication under the control of an eNB. In a case where the UE operates in D2D mode, the UE performs communication with its counterpart UE using an uplink frequency band under the control of the eNB. Further, as a prior procedure for D2D communication, the UE performs a D2D discovery procedure that allows UEs to independently exchange signals without involvement of an eNB in order to identify the presence of neighbor UEs and to determine mutual proximity.
FIG. 1 is a view illustrating an example in which a UE performs a D2D discovery operation in a communication system according to the related art.
The discovery operation by a first UE 100 and a second UE 110 is described as an example with reference to FIG. 1.
Referring to FIG. 1, the first UE 110 broadcasts discovery signals 101, 103, and 105 for informing its presence to neighbor UEs, and a neighbor UE, the second UE 110, receives the discovery signal 111. Further, the second UE 110 broadcasts discovery signals 111, 113, and 115 for informing its presence to neighbor UEs and receives the discovery signal 105 broadcast from its neighbor UE, the first UE 100.
Here, the discovery signals 101, 103, 105, 111, 113, and 115, each, are transmitted in a channel interval for D2D discovery defined by the eNB in an uplink (UL) frequency band, and the first UE 100 or the second UE 110 recognizes the presence of its counterpart UE through a discovery signal received in the channel interval defined by the eNB. The channel interval for D2D discovery is described below in greater detail with reference to FIG. 2. Hereinafter, D2D communication-applied communication system is referred to as a D2D communication system.
FIG. 2 is a view illustrating an example of the structure of a discovery channel subframe and a UL subframe used in a D2D communication system according to the related art.
Referring to FIG. 2, the discovery channel subframe 200 denotes a channel interval for D2D discovery defined by the eNB, and the UL subframe 210 is positioned continuous to the discovery channel subframe 200. The discovery channel subframe 200, assuming a 10 MHz bandwidth 202, includes multiple subframes temporally consecutive, e.g., ND subframes, over the overall frequency band. Further, the discovery channel subframe 200, in light of physical resource block (PRB), may be represented as 44 PRBs, and the discovery signal 204 is transmitted in two PRBs, as arbitrarily determined, among the 44 PRBs.
Further, a physical uplink control channel (PUCCH) signal is transmitted in three PRBs 206 and 208 adjacent to the 44 PRBs used for D2D discovery.
Meanwhile, the eNB configures a start point and end point of the discovery channel subframe 200 in the form of a system information block (SIB) so that a D2D discovery operation may be performed based on the discovery channel subframe 200 and provides the same to each UE, and each UE performs a D2D discovery operation on the basis of the time of reception of the downlink synchronization signal, e.g., a primary synchronization signal (PSS), received from the eNB.
In the cellular communication system, in a case where a UE has established a communication session with an eNB in a neighbor cell of the cell where the UE is located or the UE communicates discovery signals with a counterpart UE located in the communication coverage of the neighbor cell, the eNB in the neighbor cell should offer information on the start point and end point of the discovery channel subframe from the neighbor cell. However, frame time synchronization between eNBs is not guaranteed by nature of LTE system, and under such asynchronous network environments, a time offset Network Time Difference (NTD) up to ±1 slot (±0.5 ms) may arise.
FIGS. 3A and 3B, respectively, illustrate a system configuration and a frame structure exemplifying the occurrence of inter-cell NTD in a cellular communication system under an asynchronous network environment according to the related art.
Referring to FIGS. 3A and 3B, the cellular communication system includes a first cell 300 and a second cell 350. The first cell 300 includes a first eNB eNB1 and UEs 303, 305, 307, and 309, and the second cell 350 includes a second eNB eNB2 and UEs 353, 355, and 357. Here, assume that the UEs denoted with 303, 307, and 353 operate in Radio Resource Control (RRC) connected (RRC_CONNECTED) mode, and the UEs denoted with 305, 309, 355, and 357 operate in RRC idle (RRC_IDLE) mode. The RRC connected mode UEs, after obtaining uplink synchronization with the eNB and establishing a communication link, continue to perform communication, and the RRC idle mode UEs, after obtaining a downlink synchronization with the eNB and obtaining related system information, intermittently receive control information only.
A UE to perform D2D capability may perform a D2D discovery operation on its neighbor UEs regardless of operation modes (RRC connected mode or RRC idle mode), and the eNB may provide the UEs with information on the discovery channel position of neighbor cells, i.e., the start point and end point of the discovery channel subframe. Further, since downlink synchronization signals are received substantially at the same time in the same cell, synchronization for discovery channel subframes is ensured to some degree.
However, in a case where, in a cellular communication system under an asynchronous network environment, inter-cell discovery channel subframes are aligned based on the discovery channel position information of the neighbor cell included in the SIB, the inter-eNB NTD may become larger than the length of the cyclic prefix (CP). Further, discovery signals received from the UE included in another cell may lose their orthogonality due to such NTD, or the discovery signals received from the UE included in the other cell may interfere with a discovery signal desired to be received, presenting an obstacle to seamless D2D discovery.
In other words, as shown in FIG. 3A, discovery channel interval synchronization is guaranteed between the UEs 303 and 305 in the first cell, but not between the UE 309 in the first cell and the UE 353 in the second cell due to inconsistency in reception time between downlink synchronization signals. Further, the NTD between a subframe transmitted from the first eNB 301 and a subframe transmitted from the second eNB 351 may be represented as shown in FIG. 3B, as an example. NTDs 360, 362, and 364 up to ±slot may occur between the subframe transmitted from the first eNB 301 and the subframe transmitted from the second eNB 351. Here, assume that one subframe includes two slots, each including seven orthogonal frequency division multiplexing (OFDM) symbols.
Accordingly, the cellular communication system under the asynchronous network environment uses a separate frame synchronization scheme for scheduling the position of the inter-cell discovery channel interval in order to perform a D2D discovery operation.
FIG. 4 is a view illustrating an example of aligning inter-cell discovery channel intervals in a cellular communication system under an asynchronous network environment according to the related art.
Referring to FIG. 4, assume that the cellular communication system includes three macro cells and that the eNB of the serving cell is previously aware of per-subframe information on the start point and end point of a discovery channel for the eNB of a neighbor cell. Further, assume that the UE included in each cell may grasp the start position of the discovery channel for a neighbor cell by communicating a synchronization signal in the first subframe included in the discovery channel interval.
Referring to FIG. 4, described is a discovery channel interval aligning scheme that allocate respective discovery channel intervals for the first macro cell (Macro 1) 400, the second macro cell (Macro 2) 410, and the third macro cell (Macro 3) 420 not to overlap between the cells, for example.
That is, the UE in the first macro cell 400 communicates discovery signals in the first discovery channel interval 402 in the same way as the related art and receives discovery signals at the location 404 corresponding to the second macro cell 410 and the third macro cell 420. That is, the UE does not transmit any discovery signal at the location 404.
The UE in the second macro cell 410 communicates discovery signals in the second discovery channel interval 412 in the same way as the related art and receives discovery signals at the location 414 corresponding to the first macro cell 400 and the third macro cell 420. That is, the UE does not transmit any discovery signal at the location 414.
The UE in the third macro cell 420 communicates discovery signals in the third discovery channel interval 402 in the same way as the related art and receives discovery signals at the location 424 corresponding to the first macro cell 400 and the second macro cell 410. That is, the UE does not transmit any discovery signal at the location 424.
As shown, the first discovery channel interval 402 and the second discovery channel interval 412 are positioned with a gap for preventing overlap therebetween.
FIG. 5 is a view illustrating another example of aligning inter-cell discovery channel intervals in a cellular communication system under an asynchronous network environment according to the related art.
Referring to FIG. 5, assume that the cellular communication system includes seven macro cells and that the eNB of the serving cell is previously aware of per-subframe information on the start point and end point of a discovery channel for the eNB of each neighbor cell. Further, assume that the UE included in each cell may grasp the start position of the discovery channel for a neighbor cell by communicating a synchronization signal in the first subframe included in the discovery channel interval.
Referring to FIG. 5, the respective discovery channel intervals for the first macro cell (Macro 1) 500, the second macro cell (Macro 2) 510, and the third macro cell (Macro 3) 520 are allocated not to overlap between the cells, for example. The scheme for aligning discovery channel intervals not to overlap between cells has been described above in connection with FIG. 4, and no further detailed description thereof is given.
Then, the respective discovery channel intervals for non-neighbor cells, e.g., the first macro cell 500, a first′ macro cell (Macro 1′) 504, and a first″ macro cell (Macro 1″) 502, are allocated to overlap between the cells. In another example, the third macro cell 520, a third′ macro cell (Macro 3′) 524, and a third″ macro cell (Macro 3″) 522, may similarly be allocated to overlap between the cells. Accordingly, if the respective discovery channel intervals for the first macro cell 500, the first′ macro cell 504, and the first″ macro cell 502 are aligned, the respective discovery channel intervals of the cells are positioned in the same interval.
Techniques for aligning discovery channel intervals on an inter-cell subframe basis have been described above. However, the asynchronous network environment has an inter-cell NTD, and thus, the scheme for aligning discovery channel intervals on an inter-cell subframe basis may cause the following problems that are described below in connection with FIG. 6.
FIG. 6 is a view illustrating an example of a problem with a scheme for aligning discovery channel intervals on a subframe basis in a cellular communication system under an asynchronous network environment.
Referring to FIG. 6, assuming that UE B located in the cell where eNB B (eNBB) 610 is present receives discovery signal S 601 from UE A located in the cell where eNB A (eNBA) 600, there is an NTD between eNB A 600 and eNB B 610, and thus, a time offset larger than the CP length occurs. Accordingly, UE B, upon reception of discovery signal S 601, loses orthogonality.
Further, UE B may receive both discovery signal S 601 from eNB A 600 and discovery signal I1 621 from eNB C (eNBC) 620. However, an NTD is present between eNBs A, B, and C (600, 610, and 620, and thus, discovery signal I1 621 is received overlapping discovery signal S 601, resulting in Inter-Channel Interference (ICI).
Further, UE B may receive discovery signal I2 631 from eNB D (eNBD) 630, transmitted through a different frequency band PRB in the same subframe. However, an NTD is present between eNBs A, B, and D (600, 610, and 630), and thus discovery signal I2 631 is received by eNB B with its orthogonality lost, thus causing ICI with discovery signal S 601.
For the above reasons, signal to interference-plus-noise ratio (SINR) of discovery signal S 601 received by UE B worsens, deteriorating the UE's D2D discovery performance. Further, since the maximum size of an NTD creatable under the asynchronous network environment is ±1 slot, i.e., ±0.5 ms, the deterioration of D2D discovery performance cannot be addressed even by applying a 16.67 μs-long extended CP.
Meanwhile, the discovery channel interval aligning scheme described above in connection with FIG. 4 allows the UEs belonging to the serving cell to additionally receive discovery signals in the discovery channel interval of a neighbor cell without adjusting the position of discovery channels, preventing the above problems, i.e., orthogonality failure and ICI. However, an arising issue is that cellular communication is impossible during the interval where a discovery signal is additionally received. In particular, given a multi-cell environment, the discovery channel interval aligning scheme described above in connection with FIG. 4 employs a discovery channel interval whose length is three times or more as long as the legacy discovery channel interval (ND subframe), and in a case where the inter-discovery channel interval is considered to be up to one subframe, an additional limitation may be posed to cellular communication during subframes up to (2ND+2).
FIG. 7 is a view illustrating an example of interference incurred by a non-neighbor cell in a cellular communication system under an asynchronous network environment according to the related art.
Referring to FIG. 7, reference numeral 710 denotes a discovery radius of arrival of a discovery signal from UE A 701 located in serving cell A 700, and reference numeral 720 denotes an Intersite distance (ISD) of each cell. Considering a discovery signal with a transmit power of 23 dBm and the discovery radius 710 of about 750 under a hexagonal cell environment with the ISD 720 of 500 m, UE A 701 may receive discovery signals from a UE located in a non-neighbor cell, cell A′ 760, cell A″ 750, or cell A′″ 770. Even given a path loss due to distance, some UEs may receive discovery signals from a non-neighbor cell, and in such case, the SINR of a received discovery signal may be deteriorated.
The above information is presented as background information only to assist with an understanding of the present disclosure. No determination has been made, and no assertion is made, as to whether any of the above might be applicable as prior art with regard to the present disclosure.